Start with a wired baseline
Before changing Wi-Fi equipment, run a test over Ethernet if possible. If the wired result is healthy but wireless results are poor, the broadband line is probably not the main problem. This avoids paying for a faster package when local radio coverage is the bottleneck.
Place the router carefully
Keep the router in an open, central position and away from thick walls, metal objects, large appliances and enclosed cupboards. Raising it above floor level often helps. A router at one edge of the property has to push the signal through more obstacles to reach the opposite side.
Use the right frequency
2.4 GHz generally reaches farther but has less capacity and more interference. 5 GHz can provide much higher speeds over shorter distances. Newer Wi-Fi standards and 6 GHz equipment can add capacity where compatible devices are available.
Test before buying equipment
Measure in several rooms and note where performance drops. A well-positioned access point or wired mesh node can be more effective than a simple wireless repeater. Where practical, Ethernet backhaul gives additional access points a strong connection to the router.